Researchers from the University of Twente MESA+ research institute and the Foundation for Fundamental Research on Matter (FOM) have, as the first in the world, created thin films of a so-called 'spin ice' material. Spin ice materials are interesting for physicists, because magnetic monopole-like phenomena can occur in them. The results are published in the new scientific journal Applied Physics Letters Materials.
Magnets have a north and a south pole, which cannot be separated from one another. How often you may saw through a magnet, the result is smaller and smaller magnets with two poles. The existence of magnetic monopoles - particles with only a north or only a south pole - was already predicted in 1931, but despite intensive research by physicists it was never confirmed. Nevertheless, it is possible to perform research into magnetic monopoles using the so-called spin ice materials. In these materials it is possible to separate the magnetic poles from one another and have them move independently through the material. The magnetic charge then flows through the material, just like an electric charge can also move through a material. Spin ice materials can thus form the basis for applications in the field of 'magnetricity' (the magnetic equivalent of electricity).
Thin layers
Up till now, research into spin ice materials has always been performed with complete crystals (bulk crystals). To get closer to developing actual applications you need thin layers of the material, which you can incorporate on a chip, for example. Researchers from the University of Twente MESA+ research institute have now succeeded in creating thin films of a spin ice material. They grew holmium titanate oxide (Ho2Ti2O7) on a substrate, enabling them to create films - thin layers of between 10 and 120 nanometres thick - of the material (a nanometre is one million times smaller than a millimetre).
Spin ice
The magnetic ions (also called 'spins') within a spin ice material are forced into the crystal lattice in a tetrahedral structure (tetrahedron). Because the total magnetic moment of the material is zero, two of the magnetic moments on each tetrahedron are directed inwards and two are directed outwards. The structure is similar to frozen water, in which each oxygen atom has two short and two long bonds to four hydrogen atoms. When one of the magnetic moments on the vertex of a tetrahedron flips around, for example due to an external magnetic field, two magnetic charges occur in the material: the magnetic monopoles. If a magnetic moment on an adjacent tetrahedron turns over as well, only one of the monopoles is moved. By reversing a chain of magnetic moments, the monopoles are pulled apart, enabling them to move freely through the material.
Research
The research was conducted by Denise Leusink, Francesco Coneri, Marcel Hoek and Hans Hilgenkamp from the Interfaces and Correlated Electron systems group of the MESA+ Institute for Nanotechnology of the University of Twente and colleagues from the University of Antwerp and the Catholic University of Leuven. The research was financially supported by FOM, the Netherlands Organisation for Scientific Research (NWO) and the European Union.
Click here for the online article Thin films of the spin ice compound Ho2Ti2O7.
